1. Technical Field
The present invention relates to a light deflecting element, a light deflector equipped with a light deflecting element, and an image forming device equipped with a light deflector.
2. Related Art
In the past, there has been known a light deflector rotating a reflecting surface to thereby deflect a light beam entering the reflecting surface from a light source, with a deflected light beam scanning on an image forming surface to form an image on the image forming surface. In order for forming a high-resolution image, it is required to rotate the reflecting surface at a higher frequency. Mirror devices manufactured applying a technology of micro-electromechanical system (MEMS) can be driven at a frequency so high that polygon mirrors or galvanometer mirrors driven at that frequency can hardly be realized. The mirror device is a light deflector provided with a movable plate to which a reflecting surface is provided or attached, a torsion beam having elasticity, and a drive source, and reciprocally rotates the reflecting surface around the torsion beam as an axis to thereby deflect the light beam entering the reflecting surface from the light source.
The movable plate is required to be rotated at a high speed and with high accuracy. However, the high-speed rotation might cause deformation called “dynamic deflection” due to the inertia moment of the movable plate itself, and the dynamic deflection damages the flatness of the reflecting surface to thereby degrade the image formed.
JP-A-2005-300927 (Document 1) discloses a deflecting mirror, a light scanning device, and an image forming device, in which the distribution of the thickness of a mirror substrate (a movable plate) is set appropriately, thereby making the rigidity distribution of the mirror substrate from a rotational center beam of the mirror substrate along a direction perpendicular to the rotational axis correspond to the distribution of the bending moment, which causes the dynamic deflection, to thereby reduce the dynamic deflection of the mirror substrate, and at the same time, reduce the inertia of the mirror substrate, thus making it possible to enlarge the deflection angle.
JP-A-2003-131161 (Document 2) discloses a light deflector, a method of manufacturing the same, an optical device using the same, and a torsionally oscillating member in which at least one of the surfaces, which is located on the reverse side of the reflecting surface of the movable plate and on the both sides across the torsion shaft, is provided with a recessed section, thereby making it possible to reduce the inertial force while preventing the rigidity from decreasing.
However, the junction between the movable plate and the torsion beam is a portion for transmitting the force for deforming the torsion beam caused by shaking the movable plate and the counter force caused by the torsion beam thus deformed between the torsion beam and the movable plate, and therefore, the force the Document 1 and the Document 2 fail to discuss is applied thereto. The stress caused by this force causes the deflection the Document 1 and the Document 2 fail to discuss, which makes the deflection shape in the vicinity of the root thereof different. Therefore, there is a problem that it is difficult to sufficiently reduce the dynamic deflection of the movable plate by the method of reducing the dynamic deflection described in the Document 1 and the Document 2 alone.
Specifically, in the past, there has been a problem that the stress caused in the junction between the movable plate and the torsion beam is different between one and the other of the sides of the movable plate depending on the cross-sectional shape of the torsion beam, as a result, and the stress causes the deflection of the movable plate.